This application claims the priority of German patent document 199 11 016.6, Filed Mar. 12, 1999, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a fuel cell system of the type that can be used, for example, in fuel-cell-operated motor vehicles. In such devices, a suitable liquid or gaseous fuel mixture, such as a hydrogen-rich gas mixture or a liquid water/methanol mixture, is fed into the anode space of the fuel cell, where the fuel is electrochemically oxidized at the anode, after which the reacted mixture is removed by way of the anode discharge from the anode space. An oxygen-containing gas, such as air, is fed into the cathode space, where the oxygen is reduced at the cathode for forming water, after which the water-containing cathode waste gas is removed by way of the cathode discharge from the cathode space.
In many applications, it is desirable to be able to separate the water contained in the cathode waste gas from the other constituents, for example, to utilize once again in the system the separated water and/or the cathode waste gas purified again with respect to the contained water. For this purpose, water separating devices are arranged in the cathode discharge. Normally, these water separating devices consist of a condenser, in which the water is condensed out at a suitably low temperature, requiring a correspondingly large and voluminous heat transfer device construction for the condenser, in order to sufficiently cool down the cathode waste gas. Frequently, an expander in the form of a turbine coupled with a cathode-feed-side compressor is provided in the cathode discharge; in this case a condenser can be arranged upstream and/or downstream of the expander. Fuel cell systems of this type are described in German Published Patent Application DE 197 01 560 A1 and U.S. Patent Documents U.S. Pat. Nos. 4,001,041, 4,738,903 and 5,599,638.
The system disclosed in the last mentioned document contains a water recovery unit arranged in the cathode discharge, which provides recovered water in liquid form at its water-side outlet. That is, it is designed as a condenser and feeds the liquid water to a fuel injection unit which meters methanol as the fuel into the liquid water and dissolves it therein. The liquid methanol/water mixture is fed to a circulation tank designed as a tower and as a gas/liquid separation unit. For this purpose, it may contain tubes made of a microporous material which are penetrable by gases which are carried along (particularly carbon dioxide which was brought in with the cathode-side water), but are not penetrable by the liquid fuel/water mixture. As an alternative to the use of such microporous tubes, the liquid fuel/water mixture can be fed into the bottom area of the circulation tank, after which the gaseous carbon dioxide will then collect in the upper tank area and can be removed.
From U.S. Pat. No. 5,432,023, it is known to extract the water formed at the cathode of a fuel cell by way of a wick (which contacts the cathode and is made of a porous material), into a water recovery chamber. For this purpose, the wick is produced with a pore diameter which decreases, for example, in two stages, in the direction of the water recovery chamber, with the portion of the wick piece adjoining the cathode having a pore diameter which is smaller than that of the porous cathode material.
In the chemical esterification field, corresponding water separating membranes, usually in the form of polymer membranes, are used for water separation or water supply.
One object of the invention is to provide a fuel cell system of the type described above, which has water separating devices that can be constructed in a comparatively compact manner and permit a satisfactory separation of water contained in the cathode waste gas.
These and other objects and advantages are achieved by the present invention, in which the water separating devices characteristically contain a water separating membrane suitable for the selective water vapor separation. For this purpose, membranes are particularly suitable which are conventionally used in the chemical esterification process technique for the separating of water and for supplying water. Such a water separating membrane typically consists of a polymeric or porous material, for example, ceramics, glass or metal; and, if the pressure difference between the mixture space adjoining on the one membrane side and the water collecting space adjoining on the other membrane side is sufficiently high, it performs a satisfactorily selective water separation from the residual cathode waste gas by means of known membrane-type water separation processes.
Since, for this type of water separation, the temperature does not have to be lowered as far as required for a condensation separation, while the water separation performance of the water separating membrane is the same, it can be built in a significantly more compact and more space-saving manner than the conventionally used condensers. Even if, in addition to the water separating membrane, a condenser is provided, this condenser can be more compact than because it has only to carry out a water separating performance reduced by that of the water separating membrane unit.
In one embodiment of the fuel cell system according to the invention, an expander arranged in the cathode discharge is used for the supporting drive of a compressor arranged in the cathode feed, and contributes to the cooling of the cathode waste gas.
In another embodiment of the invention, the water separated by the water separating membrane unit can be fed back into the anode feed, and thus can be utilized on the anode side. This is expedient, for example, for systems with an anode-side water/methanol liquid circulation system in order to compensate water losses occurring there.
In still another embodiment, the water collecting space of the water separating membrane unit is adjoined by a condenser. Such a two-stage design of the water separating devices permits a particularly efficient water separation. Further, a gas recirculation pipe can be provided, which recirculates residual gas, such as residual air, which has passed through the water separating membrane and reaches the condenser, back into the cathode feed for another utilization.
According to another feature of the invention, the water separating membrane unit and the condenser may be integrated in a common constructional unit, by means of which a high water separating performance can therefore be achieved while the construction is very compact.